Neurons contain elaborate cytoskeletons which consist of microtubules, neurofilaments, and microfilaments. The neuronal cytoskeleton provides the architectural framework that defines the external shape of the neuron, and also organizes the cytoplasm to carry out motile and metabolic processes essential to life. Thus, the mechanisms for regulating cytoskeletal structure are directly involved in generating and maintaining neuronal form and function. This application proposes direct experiments to determine how cytoskeletal organization is regulated in neurons. These experiments focus on microtubules (MT) and MT proteins. The specific experiments proposed are based on previous work demonstrating that the MT system of axons differs from that of dendrites with respect to composition and organization. These findings indicate neurons generate two distinct MT systems, one for the axon and on for the dendrites. The proposed experiments will (i) identify the structures responsible for assembling and organizing the MT systems of the axon and dendrites; (ii) determine the relative roles of co-translational and posttranslation mechanisms in the compartmentation of MT proteins between the axonal and somatodendritic compartments; (iii) define how the compartmentation of MT components specializes the MT systems of the axon and dendrites. All of these experiments combine structural, immunological, and biochemical analyses of MT in intact neurons with in vitro studies of MT and MT proteins. We have already used this approach to understand how specific MT components influence the structure of the microtubule framework in neurons. Successful completion of the proposed experiments will resolve several issues concerning the regulation of MT structure and organization, and thereby further current understanding of the cellular and molecular bases of neuronal form and function. In addition, many pathologies of the nervous system are manifest at a cellular level in dramatic alterations in the neuronal cytoskeleton. By defining normal mechanisms for the control of MT organization, the proposed research will contribute toward a better understanding of these pathologies.